It is often desirable to measure mechanical strains on certain materials. The use of optical fibers for measuring strains has been employed on many occasions and in various ways. When the fibers are attached to the material under test, the physical forces that stretch, or contract the material, impart the same forces to the fiber. Stress to the optical fibers can also occur as a result of variations in temperature of the environment within which the optical fiber is located. Such physical forces and temperature variation are typically found in aerospace environments such as aircrafts or spacecrafts.
Therefore, rigidly attaching or bonding optical fibers to physical structures can measure strain to the physical structure. Optically measuring strain on the optical fiber then provides information regarding strain on the physical structure. This type of arrangement has been used, for instance, to measure stress in bridges.
Fiber Bragg Gratings have been used to optically measure strain in optical fibers. Fiber Bragg Gratings comprise a portion of the optical fiber where the index of refraction has been changed, such that it reflects a particular resonant wavelength. The Gratings are written on a section of the optical fiber, which is then spliced to longer lead-in, lead-out optical fibers. Coherent light at the resonant wavelength is transmitted down the core of the optical fiber, and reflects off the Bragg Gratings, and passes back up the fiber. The time period between transmitting the light down the core of the optical fiber and receiving the reflected light is measured.
As the grating spacing changes in response to strain, the index of refraction of the grating changes thereby altering the measured time period of modulation of the index of refraction. Multiple Fiber Bragg Grating configurations can also be used for measuring strain. In such a configuration, each Bragg Grating can have a unique central frequency (and no overlap of the frequency response). Thus, multiplexed signals can be transmitted by the optical fiber and discriminated by the Fiber Bragg Gratings.
The lasers typically used to sense strain in optical fiber use a large mechanically rotating and displacing grating element as one of the laser cavity endfaces to continuously tune the wavelength. When tuning a laser, care-must be taken to delicately control the mirror endface such that only one axial mode will be output. The tuning of the laser can also be disrupted by physical vibration, which is not uncommon on a bridge or in other locations where measuring physical strain is desirable. A DBR laser, which is a smaller, electrically controlled laser, would be less mechanically cumbersome for this purpose. However, while physically and mechanically more compact, a DBR laser can be equally cumbersome to tune.
A DBR laser has a semiconductor laser mirror endface and an optical cavity length, which are each modifiable by a current input. There are periodic states (called, “mode hops”) that the semiconductor laser mirror endface and the optical cavity length must avoid. Typically, either the semiconductor laser mirror endface is modified to tune the DBR laser or the optical cavity length is modified to tune the laser using its respective current input. The concern of mode hops and the limitations on index of refraction modification and noise concerns discourages the use of DBR lasers for applications that require continuous tuning.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.